Change proc_exit to unwind the stack rather than exiting the host process. (#1646)

* Remove Cranelift's OutOfBounds trap, which is no longer used.

* Change proc_exit to unwind instead of exit the host process.

This implements the semantics in https://github.com/WebAssembly/WASI/pull/235.

Fixes #783.
Fixes #993.

* Fix exit-status tests on Windows.

* Revert the wiggle changes and re-introduce the wasi-common implementations.

* Move `wasi_proc_exit` into the wasmtime-wasi crate.

* Revert the spec_testsuite change.

* Remove the old proc_exit implementations.

* Make `TrapReason` an implementation detail.

* Allow exit status 2 on Windows too.

* Fix a documentation link.

* Really fix a documentation link.

cranelift/filetests/filetests/parser/flags.clif

@@ -25,7 +25,7 @@ block201:
     return
 
 block202:
-    trap oob
+    trap heap_oob
 }
 ; check: v1 = ifcmp_imm v0, 17
 ; check: brif eq v1, block201
@@ -56,7 +56,7 @@ block201:
     return
 
 block202:
-    trap oob
+    trap heap_oob
 }
 ; check: v2 = ffcmp v0, v1
 ; check: brff eq v2, block201

cranelift/filetests/filetests/parser/tiny.clif

@@ -223,7 +223,7 @@ function %cond_traps(i32) {
 block0(v0: i32):
     trapz v0, stk_ovf
     v1 = ifcmp_imm v0, 5
-    trapif ugt v1, oob
+    trapif ugt v1, heap_oob
     v2 = bitcast.f32 v1
     v3 = ffcmp v2, v2
     trapff uno v3, int_ovf
@@ -233,7 +233,7 @@ block0(v0: i32):
 ; nextln: block0(v0: i32):
 ; nextln:     trapz v0, stk_ovf
 ; nextln:     v1 = ifcmp_imm v0, 5
-; nextln:     trapif ugt v1, oob
+; nextln:     trapif ugt v1, heap_oob
 ; nextln:     v2 = bitcast.f32 v1
 ; nextln:     v3 = ffcmp v2, v2
 ; nextln:     trapff uno v3, int_ovf

cranelift/filetests/filetests/regalloc/iterate.clif

@@ -139,7 +139,7 @@ block0(v0: i64):
     jump block4
 
 block4:
-    trap oob
+    trap heap_oob
 
 block2:
     v8 = load.i64 v5+8

cranelift/filetests/filetests/wasm/control.clif

@@ -54,7 +54,7 @@ block0(v0: i32):
     br_table v0, block4, jt0
 
 block4:
-    trap oob
+    trap heap_oob
 
 block1:
     return